Dialkyl tartrate-heteropolyacid developing system for dye precursor compounds

ABSTRACT

Record material for forming a durable image with a basic dye or dye precursor compound, preferably colorless, characterized by the presence on an absorbent surface thereof of a heteropolyacid reactive with the dye compound to form a colored &#39;&#39;&#39;&#39;lake&#39;&#39;&#39;&#39; or complex and a dialkyl tartrate containing about three to six carbon atoms in the alkyl groups thereof. Preferably, the heteropolyacid and dialkyl tartrate are contained in a clay-based surface coating in an amount of about 2-25 percent by dry coating weight of each. The dye compound can be applied to the record material from a transfer sheet carrying the same in an appropriate pressure-releasable state, for example, within microcapsules adhered as a layer on the sheet, and transfer sheets of this character can be manifolded with the present record material to form a pressure-sensitive record system.

United States Patent [1 1 Maalouf [451 Feb. 19, 1974 DIALKYL TARTRATE-HETEROPOLYACID DEVELOPING SYSTEM FOR DYE PRECURSOR COMPOUNDS [75] Inventor: George E. Maalouf, Niagara Falls,

[73] Assignee: Moore Business Forms Inc., Niagara Falls, N.Y.

[22] Filed: Dec. 20, 1971 [21] Appl. No.: 210,121

2,969,330 l/l96l Brynko 2,969,331 l/1961 Brynko et a1. 252/316 3,173,878 3/1965 Regen 1l7/36.8 X 3,418,250 12/1968 Vassiliades 117/36.2 X 3,418,656 12/1968 Vassiliades 117/36.2 X 3,427,180 2/1969 Phillips 1l7/36.2 3,455,721 7/1969 Phillips et a1. 117/155 X OTHER PUBLICATIONS Chemical Abstracts, Vol. 66, 1967, p. 3,733, Ser. 374lx.

Chemical Abstracts, Vol. 72, 1970, p. 101,792, ser. l0l793c.

Primary Examiner-William D. Martin Assistant Examiner-M. R. Lusignan Attorney, Agent, or Firm-William J. Daniel [5 7] ABSTRACT Record material for forming a durable image with a basic dye or dye precursor compound, preferably colorless, characterized by the presence on an absorbent surface thereof of a heteropolyacid reactive with the dye compound to form a colored lake or complex and a dialkyl tartrate containing about three to six carbon atoms in the alkyl groups thereof. Preferably, the heteropolyacid and dialkyl tartrate are contained in a clay-based surface coating in an amount of about 225 percent by dry coating weight of each. The dye compound can be applied to the record material from a transfer sheet carrying the same in an appropriate pressure-releasable state, for example, within microcapsules adhered as a layer on the sheet, and transfer sheets of this character can be manifolded with the present record material to form a pressure-sensitive record system.

20 Claims, No Drawings DIALKYL TARTRATE-I-IETEROPOLYACID DEVELOPING SYSTEM FOR DYE PRECURSOR COMPOUNDS BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to record material for recording information in the form of visible images and is concerned more specifically with a record material capable of forming visible colored images of improved durability, particularly with respect to fading upon exposure to atmospheric conditions encountered during normal use, expecially high relative humidity. The record material of the invention is well adapted for use in the socalled carbonless copying or carbon paper" systems in conjunction with one of a well-known group of preferably colorless dye-forming precursors which, upon contact with an acid reacting material, produce a distinct color formation of a colored lake or dye complex which is per se well known in the art. The dye precursor compound or other dye can be applied to the present record material in a variety of ways, a preferred one of which is by means of a transfer sheet carrying a solvent solution thereof in some pressure-releasable form, for example, within pressure-rupturable microcapsules coated as a layer thereon.

2. Description of the Prior Art A practical substitute for conventional black or dark colored carbon paper (of the type where dark colored pigment, normally carbon black is formulated in a usually waxy transfer layer as to separate onto a suitable reception material under the application of mechanical pressure applied by a typewriter key or a manually operated stylus), which provides a'substantially colorless or at most only slightly colored image transfer system, has long been a desideratum in the art and an enormous amount of research has been directed to the formulation of useful materials of this type. Because of the basic criterion of a substantially uncolored transfer material, most of the proposed new systems have depended upon the same general working phenomenon, namely, the capacity of certain colorless dye precursor compounds to participate in an electron donoracceptor reaction mechanism with an acidic compound to produce a visibly colored dye. Unfortunately, this dye-forming mechanism appears to be inherently sensitive to rapid deterioration of the dye image thus pro- 1 duced especially upon exposure to light, humidity, pH change and the like. Numerous patents have been issued describing various suggestions for obtaining dye images of improved durability in systems of this type, but much room for further improvement yet remains.

It is well known in the dye art that certain dyes, usually in the basic category, can be made to adhere to a substrate for which they are not directly substantive by means of so-called mordanting compounds which complex with both the substrate and the dyestuff and thus adhere the dyestuff onto the substrate. The complex of the dyestuff and the mordanting agent is often referred to as a lake. Among the more commonly employed mordanting agents. can be classified in two groups acid compounds such as tannic acid, and metallic compounds, such as the salts or hydroxides of such metals as aluminum, iron, chromium, tin and the like. As improved substantive dyes have been discov ered, the importance of mordant dyes for dyeing purposes has significantly declined.

It is known that certain complex inorganic acids, known as the heteropolyacids, have utility as mordanting agents or precipitating agents for basic dyes to form insoluble lakes, the more important examples of such acids being phosphomolybdic acid, phosphotungstic acid, phosphotungstomolybdic acid and phosphosilicic acid. Utilization of this capacity of heteropolyacids to mordant basic dyes in the record material field has been suggested, but this suggestion has, up to now, been only of theoretical interest. The heteropolyacids tend to be subject to decomposition on storage, especially when exposed during storage to conditions of relatively high humidity of around percent or more for periods as short as 72 hours even at room temperature and such atmospheric conditions are commonly encountered throughout much of the world. The measures that would be required to establish and maintain a protective atmospheric environment for record sheets carrying the heteropolyacid in any ordinary coating would be prohibitively expensive. The heteropolyacids also show some tendency to be attacked by exposure to light.

SUMMARY OF THE INVENTION In brief, the present invention is based on the unexpected discovery that a precise class of organic compounds heretofore used mainly as plasticizing agents in the synthetic resin field, when provided in conjunction with an image-forming system using a heteropolyacid as the acid reactant and a dye precursor compound or basic dye to form a lake or complex therewith, acts as a milieu or medium which, for reasons not at all clearly understood, contributes a substantial increase in the durability or permanence of the resultant image. This improvement was all the more surprising in view of the fact that a large number of other compounds of equal or greater worth as plasticizing agents for synthetic resins do not provide the same protective milieu as the compounds of the invention and, indeed, homologous members of the same class do not supply this valuable contribution. The improved results according to the present invention were achieved only by the presence of a dialkyl tartrate wherein the alkyl groups thereof contain only about three to six carbon atoms.

The reason for the superior performance of the dialkyl tartrates of the invention cannot be satisfactorily established. These compounds apparently act as an uniquely effective medium for the reaction between the dye compound and the heteropolyacid. This function cannot be explained in terms of a solvent action, however, because other organic compounds having an equally good solvent action do not allow the lakeor complex-forming reaction to occur. Being waterinsoluble and lacking any affinity for water, the dialkyl tartrates having about three to six carbon atoms clearly cannot serve as a source of aqueous moisture whereas the lower homologues thereof which are water-soluble give much inferior results. On the other hand, other plasticizing agents and solvents are available which do permit the actual complexing of the dye and acid to take place but in all cases these have proven highly susceptible to hydrolysis and decomposition with loss of the ability for accommodating the lake-forming reaction. Consequently, these other compounds are not capable of exerting the desirable effect of the dialkyl tartrates of this invention in protecting the heteropolyacid against deterioration by humidity, light and heat, alone or in combination, and thus prolonging the stability and aging characteristic of the receiving sheets.

GENERAL DESCRIPTION OF THE INVENTION 1. Constituent Compounds of the Invention The heteropolyacids as contemplated by the present invention may be defined as the complex acids of phosphoric acids with certain heavy metals such as molybdenum and tungsten and analogous compounds such as silicon. Specific preferred heteropolyacids include phosphomolybdic acid, phosphotungstic acid, phosphotungstomolybdic acid and phosphosilicic acid, although other analogous complex inorganic acids would be expected to perform in a generally comparable way for purposes of this invention. Interestingly enough, simple inorganic acidic compounds of the type frequently used for developing dye precursor compounds, such as zinc chloride and hydrochloric acid, have proven on testing to lack the ability to form discernible color lakes as contemplated by this invention. Thus, with the latter acidic compounds, it was not possible by means of infrared spectroscopy to confirm the formation of a color lake with any dye compound of the invention. Consequently, the utilization of one or more of the heteropolyacids as defined above is considered to be an indispensible requirement of this invention.

As indicated above, only the dialkyl tartrates which contain about three to six carbon atoms in the alkyl groups thereof have been found effective to give the improved results which underlie this invention. As will be demonstrated by the experimental results in the subsequent description of working embodiments, the dimethyl and diethyl homologues are inferior. The longchain homologues are not available for testing from any commercial source but are not considered likely to give satisfactory performance. Within the range of three to six carbon atoms, the dibutyl and diisoamyl esters have been found particularly effective and are preferred. The present tartrate esters exhibit optical activity and exist theoretically in both the dextro and laevo isomeric forms. All of the examples tested here were in the dextro form but no reason is known why the L-form should not give generally equally acceptable results if available.

The dialkyl tartrates of the tape in question appear to exert a dual function in the system of concern here. First, these compounds have a protective effect on the heteropolyacids present in the record material so as to The colorless dye precursor compounds are the preferred colorant ingredient employed in this invention and the requirements of the invention as to the choice of this compound do not appear to diverge significantly from the systems of the prior art which depend upon an electron donor-receptor image-forming mechanism. In general, many if not all colorless dye precursor compound known to be effective for purposes of this mechanism in the prior art would be expected to be equally useful here. The significance of the term colorless as applied to the precursor compounds of the invention should not be over-emphasized as the initial color of the dye precursor compound is by no means of critical importance and is determined primarily by practical considerations dependent upon the appearance desired for the particular record material being manufactured. Obviously, the starting color of the precursor compound should differ significantly from the ultimate color developed in the color lake image in order for good image differentiation to be achieved. For most purposes, precursor compounds which are substantially free of any color will be found preferable from the standpoint of aesthetic appeal, but compounds with light tints or hues can ordinarily be employed without any serious objection. In the light of these circumstances, the term colorless as employed here should be shield and stabilize the heteropolyacid against atmospheric attack and degradation to which the heteropolyacids would otherwise be strongly susceptible. Therefore, the record material sheets embodying the present improvement will be useful over longer periods of time, i.e., possess a greater storage or shelf life, compared to similar material omitting the dialkyl tartrate. Secondly, these compounds promote the complexing or lake-forming reaction itself between the dye compound and heteropolyacid. The resultant lake products of typical instances of such reactions in the presence of one of the instant dialkyl tartrates have been extracted, examined and found to have an especially beneficial combination of properties including intense coloration, insolubility in most organic solvents, and good stability to heat, light as well as moisture.

interpreted in a relative sense as denoting the absence of any coloration that would correspond too closely with, or otherwise fail to sufficiently contrast with, the color of the color lake forming the ultimately developed image.

Specific precursor compounds which have been employed with good results in specific embodiments of the present invention include the morpholino derivatives of Michlers hydrol, Crystal Violet lactone, Crystal Violet carbinol, benzyl ether of Crystal Violet carbinol, ethyl Among such patents may be mentioned U.S. Pats. Nos.

2,505,470 and 3,455,721 and Canadian Pat. No. 768,039 and the disclosure thereof with respectto the precursor compounds is incorporated herein by reference.

While a colorless dye precursor compound is preferred in the practice of the invention, the presence of such a compound is not an indispensible requirement, at least in principle, of the practice of the invention. Many of the usual basic dyes which do form lakes or are precipited with heteropolyacids exist in a salt form,

and images of these complexes will be benefited in the same way by the invention. Parenthetically, note may be taken of the fact that the dye precursors mentioned above are mainly of basic dyes. When used in dye form, the image-forming ingredient will, of course, be more or less strongly colored and thus will present the disadvantages inherent in this property, but where these disadvantages are not objectionable or can be dealt with in other ways, as by the provision of a masking agent in the transfer material, the dye form will produce an equally useful and improved imaged.

The dye or colorless dye precursor compound, as the case may be, is dissolved in a suitable solvent to facilitate application thereof to the record material of the invention. Preferred solvents include xylene, dibutylphthalate and mineral oil, although a variety of others could be substituted if desired. These could be selected from a number of different groups including the natural oils, such as castor oil, kerosene and the like; synthetic oils including dichlorodiphenyl, chlorinated paraffin and the like; ester oils such as diethyl phosphate, dicresyl phosphate, ethyl and butyl acetate and the like; aromatic solvents including benzene, cresol, etc.; and the chlorinated hydrocarbons such as methylene chloride, tetrachloroethane, etc. Solvents which are substantially hydrophobic or non-polar should be employed and relatively low volatility is an advantageous characteristic.

In general, organic, non-polar solvents are preferred since, in the main, they tend to evaporate quickly with minimum effect on the record sheet, are at least adequate solvents for the dye compounds employed in the invention, especially the preferred dye precursor c0mpounds, and are readily encapsulated in available encapsulation systems. Under appropriate circumstances, aqueous media and/or polar solvents might well be useful for the dye compounds soluble therein. Volatile liquids such as xylene tend to dry quickly and resist possible image smudging but less volatile oils such as dibutyl phthalate and mineral oil give good durable images and any tendency to smudge can be overcome in other ways, as by means of an absorbent coating, containing finely divided clay, for example, on the record material.

The amount of the precursor compound dissolved in the solvent may vary dependent upon the particular materials selected for this purpose and its effectiveness in the image-forming mechanism. Minimum amounts will be normally most desirable because of the high cost of the precursor compound. In general, a concentration of about 1-6 percent by weight of the organic solvent solution. thereof will be found to give acceptable results.

The heteropolyacid and the dialkyl tartrate ester are preferably provided in conjunction with a clary coating applied to the surface of a suitable support, such as paper and the like, to make the record material. In accordance with this invention, the clay does not participate in the image-forming mechanism, in contrast to many other generally similar record systems of the prior art and, consequently, the clay component can be, and preferably is, totally inert with respect to the rest of the components of the system. This is an important feature of the invention as it makes possible the use of clays which confer maximum whiteness upon the coatings of the invention and also allows important pragmatic considerations such as cost to be taken into account in selecting the particular clay to be employed. Clays of the so-called Hydral series have provenparticularly useful. These clays are constituted of hydrated aluminum oxide and are characterized by extremely fine uniform particle size and a fluffy snowwhite appearance. They are available in several grades of which the high white grade gives the best whiteness. Other inert clays could readily be substituted, such as kaolin.

To avoid chalking or powdering of the clay coating during use, the addition of a minor amount of an organic binding agent to the clay coating composition is a desirable measure as is commonly practiced in the clay-coating art. The effective binder is starch, such as oxidized starch, specifically that sold under the tradename Stayco M starch supplied by the A. G. Staley taining approximately 2040 percent by weight of clay in aqueous suspension together with an effective binding amount of organic binding agent such as starch. Based on this formulation, the content of the heteropolyacid and tartrate ester may each vary between about l-12.5 percent by weight with the range of 2-7.5 percent being preferred. Converted to dry coating weight on the basis of an approximately 40 to 60 percent solids in the coating, this range becomes approximately 225 percent by weight for each of the components in question. Thus, on a dry basis per unit coating weight per unit area of the sheet surface, a preferable range for the major constituents will be approximately 40 to percent clay, 10 to 20 percent binder,

2 to 25 percent for each of the heteropolyacid or the dialkyl tartrate. Coating weight could range between 2-l 2 gm/sq.m. of surface and a preferred range is 4-10 gm/sq.m. of surface, all on a dry weight basis.

The application of the heteropolyacid and the tartrate ester in a clay coating is not critical to the practice of the present invention as these compounds could be impregnated directly onto the surface of any absorbent substrate appropriate for record material purposes with generally acceptable results. In this connection it is to be noted that the plasticizing ester will ordinarily have the consistency of a light liquid and is an effective medium for the heteropolyacid. It is also miscible with common organic solvents, oils and hydrocarbons and could be diluted by the addition of one of the latter if needed.

In principle, the present invention is not dependent upon any particular mode of applying the dye compound to the surface of the record material carrying the heteropolyacid and the tartrate ester and a variety of different techniques would be suggested to the skilled worker in the art for contacting the dye compound in image-wise fashion on, the record material. As mentioned, a preferred way of applying the dye compound is by way of a transfer sheet carrying on one of its surfaces a coating containing the dye compound in a form that is releasable when the transfer sheet is subjected to marking pressure either during writing with a pen or stylus, under the impact of a typewriter key, or otherwise. Solutions of colorless dye precursor compounds are preferred and lend themselves especially well to use in conjunction with virtually any of the known constructions of transfer sheets employed heretofore for similar record systems and a particularly desirable type of transfer sheet has the precursor compound contained within microcapsules distributed in the form of an adherent layer on an appropriate supporting sheet, such as paper, a plastic film, or the like.

Numerous patents have issued in the past several decades disclosing different techniques for making microcapsules having various combinations of properties from various reagents. Virtually any of these techniques could be adapted to the encapsulation of solutions of the present dye precursor compounds, subject only to the obvious condition that the microcapsule wall-forming material itself and the solvent liquid con-.-

tained therein be compatible with the remainder of the image-forming system of this invention. To identify only a few of the patents to which reference might be made for the particulars of procedures for forming microcapsules useful in the present context, reference may be made to the Green et a1. Pat. Nos. 2,800,457 and -458, and the Brynko et a1. Pats. Nos. 2,969,330 and -331, plus a considerable number of improvement patents issued thereon, as well as the Bassiliades Pats. Nos. 3,418,250 and -656 plus several patents to Reys, such as No. 3,173,878. A preferred microencapsulation procedure is that described in the Ruus Pat. No. 3,429,827, commonly assigned with the present application, wherein the capsule walls are formed by means of an interfacial condensation polymerization reaction between polymerization reactants carried in the respective phases of a two-liquid phase system. The dye precursor solution of the invention is dissolved in the dispersed phase liquid so as to become enclosed within the capsule walls ultimately produced around the dispersed phase globules.

DESCRlPTlON OF WORKING EMBODIMENTS 1. Effect on Image Light Fastness of Dialkyl Tartrate of the invention In order to test the effectiveness of the present invention, experimental systems were prepared including a transfer sheet and a reception or record sheet. To prepare the transfer sheet, a xylene solution of a particular precursor compound, e.g., the methyl ether of Michlers hydrol, at a concentration of 3.3 percent was encapsulated in accordance with the above identified Ruus patent by the condensation polymerization of teraphthalic acid chloride and a mixture of ethylene diamine and diethylene triamine, using the following procedure: in 5.0 grams of the given solvent, e.g., xylene, is dissolved 0.5 gram of trimesoyl chloride and 1.2- grams of terephthaloyl chloride. 0.5 grams of the selected precursor compound are dissolved in 10.0 grams of the same solvent, and the two solutions are mixed. The resultant solution is then emulsified into 50 ml of water containing 0.5 percent polyvinyl alcohol, after which a solution of 10 ml water, 0.9 grams diethylene triamine, 0.4 grams ethylene diamine and 0.7 grams so-' dium carbonate is added to the emulsion. The emulsion is stirred at a slow speed for approximately 3 hours at about room temperature to allow the microcapsule wall-forming reaction to reach substantial completion and the resultant microcapsules can then be used directly for coating the transfer sheet. The xylene solution constituted approximately percent by weight of the dry microcapsules and the microcapsules were applied to one surface of a paper sheet at a coating weight of about 2.8 grams/sq.m. of surface area.

Reception sheets were prepared using a combination of phosphotungstic acid or phosphomolybdic acid with dibutyl tartrate or with triacetin, a known plasticizing agent included for sake of comparison, as well as with the same acids without any plasticizing agent, also for comparison. For preparing these various reception sheets, a basic clay coating composition was utilized containing 35.4 percent of a 25 percent oxidized starch solution (Stayco M starch), 35.4 percent of finely divided clay (Hydral 705), about 0.2 percent of the sodium salt of a polymeric carboxylic acid as a dispersing agent (Tamol 850), and the balance of 29 percent of water. The starch solution was added to the water containing the dispersing agent and mixed for about 10 minutes after which the clay was added slowly under conditions of high shear mixing until a smooth homoge.

neous blend was obtained. To this basic clay formulation, various predetermined percentages of the other ingredients of the reception sheet were added as indicated in the hereinafter appearing tabulation. The clay coating is applied to a paper substrate and dried at about C for 1 minute to give a dry coating weight of about 7 grams/sq.m. of surface area.

Test images were produced using the just-described materials by placing the transfer sheet with the microcapsule layer face down upon the reception sheet and marking an approximately l-square centimeter area with a soft pencil to obtain good transfer of the precursor solution from the ruptured capsules onto the face of the underlying reception material. After examination for optical density of the initial or starting image, these test images were exposed to laboratory lighting at the distance of about 22 inches for extended periods of time and were periodically observed as to optical density to determine loss of image density over these periods. The image density readings, both initial and subsequent, were taken by means of a Macbeth densitometer, Model RD-lOO, following the instructions set forth in the Owners Manual supplied by the manufacturer of that instrument. For these light fading tests, the lights employed were 96 inch cool white fluorescent lamps with a rating of watts and a T-l2 bulb size and the temperature and humidity were those normally ambient in the laboratory during this period. In order to compare the effect of pre-exposure of the record sheets to high relative humidity, two sets of recording sheets were used for these image tests a first set which was not exposed to high humidity before being used for image development and a second set which was exposed preliminarily to a high humidity atmosphere (86 percent relative humidity) at 26C for 75 hours before being used for image development. The results of these light fading tests are set forth in the following tabulation:

TABLE 1 EFFECT ON LIGHT RESISTANCE OF DIALKYL TARTRATE VS TRlACETlN A. Un-Humidificd Sheets IMAGE DENSITY READINGS Sheet Coating B. PrcHumidif|cd Sheets IMAGE DENSITY READINGS Sheet Coating Containing lnitial After 1 Mo. After 2 Mos.

3.5% ca PTA/DBT 0.82 0.48 0.35

3.5% ca PTA/TA 0.40 0.10 0.08

3.5% PTA alone 033 0.09 0.07

10.0% PTA 5% DBT 0.90 0.85 0.74

10.0% PTA 5% TA 0.65 0.16 0.11

10.0% PTA alone 067 0.18 0.12

Initial Days After 2 A Mos.

3.5% ca PMA/DBT 0.76 0.52 0.40 7 M 3.5% PMA alone 0.57 0.26 0.18 10.0% PMA 5% DBT 0.96 1.01 0.98

10.0% PMA alonc 0.85 0.67 0.59

PTA Phosphotungstic Acid DBT Dibutyl Tartratc TA Triacctin As the aforegoing values establish, the tartrate ester of the invention strongly prolongs the light fastness of images es vf m he W929... "P595295: heteropolyacid reaction especially at relatively low concentration levels. At such low concentration levels, the comparative plasticizing agent triacetin performed little better t tbil s erepq y sis a aass ssiallx where the sheets were pre-humidified prior to image development. At substantially high concentration levels of the heteropolyacid as well as the plasticizing agent, the triacetin conferred some degree of improve: ment, at least where the sheets were not subjected to a humidified atmosphere in advance but such improvement was not comparable to that supplied by the tarirate .estcrof.the..inyentioa..w v a 2. Effect of Concentration and Carbon Content of Dialkyl Tartrate on Light Fastness of Various Dye Images e In order to determine the effect on the light fastness of the obtained dye images of variation in the chain length of the alkyl groups of the dialkyl tartrate esters of the invention, as well as of the concentration level of usage of this ingredient plus the heteropolyacid, additional series of tests were run in the same general manner as described above using three different colorless dye precursor solutions, namely, the methyl ether of Michlers hydrol dissolved in xylene at a concentra- -tion of 3.3 percent, the bis(dimethylaminophenyl) morpholinoyl methane in mineral oil at a concentration of 3.3 percent, and the benzyl ether of Michlers hydrol in xylene at a concentration of 3.3 percent. Record of developing sheets were prepared to contain three different concentration levels of phosphotungstic acid alone and in combination with four tartrate esters differing only as to the number of carbon atoms in the alkyl groups thereof at the same three concentration levels. One set of such sheets was prepared without prehumidification while another set was subjected to severe pre-humidification in the same manner as explained in SA above prior to utilization of the same for image development. Image density readings were obtained initially, after 1 days exposure since for certain precursor compounds a significant period of time is required for maximum color development before fading begins, and then after 10 and 20 days exposure to laboratory lighting as defined above (except where other exposure periods are occasionally specified), using the same density reading procedure as before. The results of these tests are summarized in the following tabulations separated according to the particular precursor solution employed.

TABLE 2 1 TABLE 2A PRECURSOR SOLUTION METHYL ETHER OF MICHLERS HYDROL IN XYLENE A. Un-Humidificd Sheets IMAGE DENSITY READINGS Sheet Coating After After After Containing Initial 1 Day Days Days 2.5% ea PTAIDMT 0.38 0.29 0.15 0.11 2.5% ca PTA/DIET 0.42 0.32 0.16 0.12 2.5% ca PTA/DBT 0.47 0.43 0.28 0.21 2.5% ca PTA/DIAT" 0.62 0.52 0.31 0.25

3.5% PTA alone 0.46 0.36 0.18 0.14 3.5% ca PTA/DMT 0.54 0.48 0.26 0.19 7 3.5% cu PTA/DET 0.54 0.46 0.24 0.18 3.5% ca PTA/DBT 0.68 0.64 0.52 0.44 3.5% ca PTA/DIAT 0.66 0.64 0.50 0.42

10% PTA alone 0.68 0.62 0.42 0.34 10% PTA 5% DMT 0.72 0.68 0.48 0.39 10% PTA 5% DET 0.75 0.72 0.56 0.49 10% PTA 5% DBT 0.68 0.74 0.70 0.66 10% PTA 5% DIAT 0.78 0.83 0.77 0.72

B. Prc-Humidificd Sheets 2.5% ca PTA/DMT 0.40 0.30 0.14 0.10 2.5% ca PTA/DET 0.36 0.28 0.14 0.10 2.5% ca PTA/DBT 0.42 0.34 0.18 0.12 2.5% ca PTA/DIAT 0.47 0.42 0.23 0.16

3.5% PTA alone 0.42 0.36 0.16 0.11 3.5% ca PTA/DMT 0.52 0.42 0.20 0.13 3.5% ea PTA/DET 0.48 0.37 0.18 0.12 3.5% ca PTA/DBT 0.54 0.50 0.32 0.24 3.5% ca PTA/DlAT 0.55 0.56 0.42 0.34

10% PTA alone 0.76 0.74 0.44 0.32 10% PTA 5% DMT 0.58 0.56 0.32 0.23 10% PTA 5% DIET 0.66 0.62 0.36 0.27 10% PTA 5% DBT 0.72 0.80 0.78 0.72 10% PTA 5% DlAT 0.70 0.80 0.73 0.67

PTA Phosphotungstic Acid DMT Dimcthyl Tartratc DET Diethyl Tartratc DBT Dibutyl Tarlralc "DIAT Dii soamyl Tartrate ERALOIL TABLE 2B.PRECURSOR SOLUTION B1S(DIMETHYL- AMINOPHENYL) MORPHOLINOYL METHANE 1N MIN? ERAL-OIL-Continued Image density readings After Image density readings After After 5 5 After After After Sheet coating containing Initial I day 10 days 20 days Sheet coating containing Initial 1 day 10 days 20 days B. Pre-humidified sheets:

A. Un-humidified sheets: 2.5% ea PTA/DMT .30 .22 .08 .08 2.5% ea PTA/DMT 0.24 0.18 0.09 0.08 2.5% ea PTAIDET.. .28 .22 .09 .08 2.5% ea PTA/DET.... .26 .20 .09 .07 2.5% ea PTA/DBT.. .34 .28 .11 .09 2.5% ea PTA/013T... .44 .38 .20 .16 25% ea PTA/DIAT. .44 .38 .19 .14 2.5% ea PTA/D1AT.. .41 .36 .20 .16 3.5% PTA alone .31 .24 .10 .08 3.5% PTA alone .26 .21 .10 .08 3.5% ea PTA/DMT.... .44 .38 .14 .10 3.5% ea PTA/DMT... .33 .26 .10 .07 3.5% ea PTA/DIET. .38 .28 .10 .08 3.5% ea PTA/DET.... .36 .30 .11 .08 3.5% ea PTA/DBT.. .50 .44 .26 .20 3.5% ea PTA/DBT.... .46 .44 .30 .26 3.5% ea PTA/DIAT. .52 .48 .31 .25 3.5% ea PTAIDIAT.. .58 .45 .40 .36 10% PTA alone.... .62 .60 .24 .16 10% PTA alone .56 .49 .23 .16 10% PTA alone... .62 .60 .24 .16 10% PTA 5% DMT... .60 .52 .28 .21 10% PTA 5% DMT. .62 .53 .26 .18 10% PTA 5% DET... .66 .58 .34 .26 l 10% PTA 5% DET.. .58 .51 .24 .16 10% PTA 5% DBT... .70 .69 .62 .58 10% PTA 5% DBTN .68 .68 .58 .53

, .LQ% .B.TA-Z ...D1AI........... .79. ,9 .13 .54 10% PTA 5% DIAT .66 .67 .58 .53

' TABLE 2c A. UnHumidified Sheets IMAGE DENSITY READINGS After Sheet Coating After After Containing Initial 1 Day 7 Days 16 Days 2.5% ca PTA/DMT 0.38 0.32 0.18 0.12 2.5% ea PTA/DET 0.30 0.26 0.14 0.10 2.5% ca PTA/DBT 0.38 0.38 0.24 0.18 2.5% ca PTA/DIAT 0.40 0.41 0.27 0.21

3.5% PTA alone 032 0.27 0.14 0.10 35% ca PTA/DMT 0.39 0.34 0.17 0.12 3.5% ea PTA/DET 0.42 0.36 0.19 0.12 3.5% ea PTA/DBT 0.61 0.64 0.52 0.45 3.5% ca PTAIDIAT 0.58 0.60 0.49 0.41

10% PTA alone 0.62 0.58 0.38 0.29 10% PTA 5% DMT 0.66 0.62 0.44 0.34 10% PTA 5% DET 0.66 0.62 0.44 0.34 10% PTA 5% DBT 0.76 0.81 0.80 0.78 10% PTA 5% DlAT 0.76 0.82 0.80 0.75

B. Pre-Humidified Sheets M... After After Initial 3 Days 7 Days 13 Days 2.5% ea PTA/DMT 0.28 16 0.12 0.09 2.5% ea PTA/DET 0.24 0.14 0.11 0.09 2.5% ca PTA/DBT 0.30 0.20 0.15 0.12 2.5% ea PTA/DIAT 0.34 0.25 0.20 0.17

3.5% PTA alone i 0.32 0.18 0.13 0.10 3.5% ca PTA/DMT 0.40 0.24 0.18 0.13 3.5% ea PTA/DET 0.36 0.21 0.16 0.13 3.5% ca PTA/DBT 0.44 0.32 0.25 0.20 3.5% ca PTA/DIAT 0.56 0.47 0.40 0.35

10% PTA alonc 0.56 0.38 0.29 0.20 10% PTA DMT 0.50 0.34 0.26 0.20 PTA 5% DET 0.55 0.36 0.27 0.20 10% PTA 5% DBT 0.74 0.75 0.74 0.72 10% PTA 5% DlAT 0.74 0.76 0.74 0.72

fie delinite superiority of theEli lift y wl tartrate and the diisoamyl tartrate, i.e., containing four and five carbon alkyl groups respectively, over the dimethyl and diethyl tartrates containing one and two carbon alkyl groups, respectively, is consistently demonstrated throughout the results of the above-tabulated tests every at very low concentration levels of 2.5 percent of each of the phosphotungstic acid and the tartrate ester. In general, the homologous esters of lower carbon content were found to be little more effective than the phosphotungstic acid alone, especially in conjunction with sheets subjected to pre-humidification prior to image formation. In sharp contrast, the improved image fastness of the images produced in the presence of the tartrate esters contemplated by the present invention persisted with at most only slight change even when obtained with pre-humidified sheets.

Composite record material consisting of a single sheet containing the image-forming components on a common surface thereof is a well-known variant in the art and the invention can be readily adapted to this embodiment. Thus, the needed quantity of microcapsules could be added to the claycoating formiilation described above and the resultant mixture applied to a sheet of paper or thelike. When pressure is brought imagewise against this sheet, the capsules rupture in the image area and react with the adjacent ingredients to produce a visible dye image. Other alterations and modifications available in this art could also be obviously employed in the practice of this invention.

I For all of the advantages described above to be achieved in the practice of the invention, the tartrate ester should be present in the record material carrying the heteropolyacid. However, if stability of the record material against deterioration by aging is not important under particular circumstances, the tartrate 'ester could be provided via the transfer sheet to be applied to the record material with the dye compound and still obtain valuable benefits in terms of the formation of a strong and durable image of the dye complex or lakelqTrimEWylch c diHityrate 7 1' 1671555505 7 r 2. Trioctanion l7. Monoacetin 3. Triethylene glycol diueetatc 18. Caprolactam 4. Oxybis (2-ethyl) acetate 19. Butanol 5. Dimethyl acetamide 20. lso-Deeanol 6. Dimethyl su'lfoxide 21. l Hcxanol 7. Pentaerythrityl tetracetatc 22. lsopropanol 8. 1,2,4 Phenyl triaeetate 23. Di-isobutyl c'arbinol 9. N,N-Diethylccyclohexylaminc 24. Methoxytriglycol acetate 10. Sucrose benzoate 25. Carbowax 400 ll. Di-N-Butyl succinatc 26. N-Hexyl carbinol l2. Di-N-Butyl adipate 27'. Dibutyl carbitol l3. Pentyl acetate lsopentyl acetate 15. 2-Cyclohcxylechlohexanol Glyceral Hexyl ecllosolve N,N-Diethyl formamid and its homologues What is claimed is:

l. The method of producing on a record material a dye image of improved durability from a dye compound forming a lake with a heteropolyacid which comprises bringing a dye precursor compound into im-' agewise contact with a record material carrying a heteropolyacid at its surface in the presence of a dialkyl tartrate containing about 3-6'carbon atoms in the alkyl groups thereof. 7

2. The method of claim 1 wherein said dye precursor compound is applied as an organic solvent solution.

3. The method of claim 1 wherein said heteropolyacid is present in an absorbent coating on said record material in an amount of about 2-25 percent by dry coating weight.

4. The method of claim 1 wherein said dye compound is of a colorless dye precursor compound.

5. The method of claim 4 wherein the dye compound is the colorless precursor of a basic dye. 4

6. The method of claim 3 wherein said dialkyl tartrate is present in said absorbentcoating in an amount of about 2-25 percent by dry coating weight.

7. The method of claim 4 wherein said dye precursor compound is applied in an organic solvent solution at a concentration of about 1-6 percent by weight.

8. Record material for forming an image of improved durability with a dye compound forming a lake with a heteropolyacid comprising a support having an absorbent surface and uniformly distributed over such surface an image-forming amount of a heteropolyacid and a minor amount of a dialkyl tartrate wherein the alkyl groups contain about three to six carbon atoms.

9. The record material of claim 8 wherein said record material has a coating thereon of an inert clay in finely divided form and said heteropolyacid and said dialkyl tartrate are present in said coating.

10. The record material of claim 9 wherein said heteropolyacid and dialkyl tartrate are each present in an amount of about 2-25 percent by dry weight of said coating.

1]. The record material of claim 9 wherein said clay is distributed in a permeable acid-insensitive organic binding agent.

12. The record material of claim 1] wherein the amount of said clay is about 4080 percent and sufficient binding agent up to about 20 percent is present to adhere said clay to the sheet surface, all percentages being by dry coating weight.

13. A pressure-sensitive record system comprising a manifold set of at least one transfer sheet and at least one receiving sheet, said transfer sheet carrying on at least a portion of a surface thereof in an insulated pressure-releasable state a solution of a dye compound adapted to produce a lake on contact with a heteropolyacid,-and said receiving sheet carrying on at least a corresponding area of an adjacent absorbent surface thereof a heteropolyacid and a dialkyl tartrate having about three to six carbon atoms in the alkyl groups thereof.

14. The record system of claim 13 wherein said dye compound is a colorless organic dye precursor compound. M

15. The record system of claim 13 wherein said record material carrys on said surface a coating containing about 40-80 percent clay and about 2-25 percent of each of said heteropolyacid and said dialkyl tartrate, all percentages being by dry coating weight.

16. The record system of claim 13 wherein said dye compound solution is contained in microcapsules forming a layer on said transfer sheet.

17. The record system of claim 14 wherein said dye precursor compound is present in a concentration of about l-6 percent by weight.

l8. The record system of claim 16 wherein said microcapsule layer has a weight of about l-l2 gm/sq.m. of surface area.

19. The record system of claim 15 wherein the total dry' weight of said coating is about 2-l2 'gm/sq.m. of surface area.

20. The record system of claim 9 wherein the total dry weight of said coating is about 2-12 gm/sq.m. of

surface area. 

2. The method of claim 1 wherein said dye precursor compound is applied as an organic solvent solution.
 3. The method of claim 1 wherein said heteropolyacid is present in an absorbent coating on said record material in an amount of about 2-25 percent by dry coating weight.
 4. The method of claim 1 wherein said dye compound is of a colorless dye precursor compound.
 5. The method of claim 4 wherein the dye compound is the colorless precursor of a basic dye.
 6. The method of claim 3 wherein said dialkyl tartrate is present in said absorbent coating in an amount of about 2-25 percent by dry coating weight.
 7. The method of claim 4 wherein said dye precursor compound is applied in an organic solvent solution at a concentration of about 1-6 percent by weight.
 8. Record material for forming an Image of improved durability with a dye compound forming a lake with a heteropolyacid comprising a support having an absorbent surface and uniformly distributed over such surface an image-forming amount of a heteropolyacid and a minor amount of a dialkyl tartrate wherein the alkyl groups contain about three to six carbon atoms.
 9. The record material of claim 8 wherein said record material has a coating thereon of an inert clay in finely divided form and said heteropolyacid and said dialkyl tartrate are present in said coating.
 10. The record material of claim 9 wherein said heteropolyacid and dialkyl tartrate are each present in an amount of about 2-25 percent by dry weight of said coating.
 11. The record material of claim 9 wherein said clay is distributed in a permeable acid-insensitive organic binding agent.
 12. The record material of claim 11 wherein the amount of said clay is about 40-80 percent and sufficient binding agent up to about 20 percent is present to adhere said clay to the sheet surface, all percentages being by dry coating weight.
 13. A pressure-sensitive record system comprising a manifold set of at least one transfer sheet and at least one receiving sheet, said transfer sheet carrying on at least a portion of a surface thereof in an insulated pressure-releasable state a solution of a dye compound adapted to produce a lake on contact with a heteropolyacid, and said receiving sheet carrying on at least a corresponding area of an adjacent absorbent surface thereof a heteropolyacid and a dialkyl tartrate having about three to six carbon atoms in the alkyl groups thereof.
 14. The record system of claim 13 wherein said dye compound is a colorless organic dye precursor compound.
 15. The record system of claim 13 wherein said record material carrys on said surface a coating containing about 40-80 percent clay and about 2-25 percent of each of said heteropolyacid and said dialkyl tartrate, all percentages being by dry coating weight.
 16. The record system of claim 13 wherein said dye compound solution is contained in microcapsules forming a layer on said transfer sheet.
 17. The record system of claim 14 wherein said dye precursor compound is present in a concentration of about 1-6 percent by weight.
 18. The record system of claim 16 wherein said microcapsule layer has a weight of about 1-12 gm/sq.m. of surface area.
 19. The record system of claim 15 wherein the total dry weight of said coating is about 2-12 gm/sq.m. of surface area.
 20. The record system of claim 9 wherein the total dry weight of said coating is about 2-12 gm/sq.m. of surface area. 